1. Field of the Invention
The present invention relates to a drive apparatus which is used in conjunction with a vibrating-type compressor and is configured to supply electrical power to the compressor by use of an inverter, and, more particularly, to a drive apparatus for a vibrating-type compressor which enables use of both AC and DC power sources without use of a mechanical changeover unit.
2. Description of the Related Art
Conventionally, an AC/DC power supply apparatus as shown in FIG. 14 (from Japanese Patent Applications Laid-Open (kokai) No. 7-111781) has been used as a drive apparatus for a vibrating-type compressor. In FIG. 14, reference numeral 1 denotes a vibrating-type compressor which is used as part of a refrigerator and operates upon receipt of a low AC voltage of, for example, 12 V or 24 V. Reference numeral 2 denotes a DC power source such as a battery which is mounted on a vehicle and which outputs a DC voltage of 12 V or 24 V.
Alternating current from a commercial AC power source 10 is converted to direct current by an AC/DC converter 8 in order to obtain a DC voltage equal to the DC voltage E obtained from the battery 2. One of the DC voltage E output from the battery and the DC voltage output from the AC/DC converter 8 is selected by an automatic changeover unit and is converted to an AC voltage by an inverter 6. The thus-obtained AC voltage is supplied to the vibrating-type compressor 1. The inverter 6 includes a first transistor 52 and a second transistor 53. These transistors 52 and 53 are turned on alternately in order to generate the AC voltage.
The voltage E output from the battery 2 or the AC/DC converter 8 is applied to the first transistor 52, whereas a voltage -E output from a polarity inversion circuit 3 is applied to the second transistor 53. The polarity inversion circuit 3 includes a transistor 11, a pulse generation circuit 12, a choke coil 13, a diode 14, and a capacitor 15 and outputs a DC voltage xe2x88x92E, whose polarity is opposite the DC voltage E with respect to ground.
A control unit 7 variably controls the AC voltage output from the inverter 6 by means of changing the duty ratios of respective output waveforms of the first and second transistors 52 and 53, to thereby change the frequency of the AC voltage applied to the vibrating-type compressor 1.
The resonance frequency of the vibrating-type compressor 1 changes depending not only on variation in load but also on the environment in which the vibrating-type compressor 1 is used. Therefore, if the frequency of the AC voltage supplied to the vibrating-type compressor 1 is maintained constant, the efficiency of the vibrating-type compressor 1 is low. In view of this problem, there has conventionally been used a technique for controlling the frequency in order to minimize the difference between the first-half peak and the second-half peak within each period of the waveform of current flowing through the vibrating-type compressor 1, to thereby maximize the efficiency of the vibrating-type compressor 1.
A frequency following circuit 24 shown in FIG. 14 compares the average value of current flowing through a shunt resistor 20 during a first half of a single waveform period of an oscillation signal output from an oscillator 21 and that during a second half of the period and outputs a control signal for variably controlling the oscillation frequency of the oscillator 21 such that the difference assumes a predetermined value. Accordingly, the oscillator 21 generates a pulse signal of a frequency corresponding to the control signal, which pulse signal is then subjected to frequency division effected by a frequency divider 22. A pulse signal output from the frequency divider 22 is supplied to a transistor control circuit 23 in order to control the first and second transistors 52 and 53. Therefore, the power supply apparatus can generate AC voltage whose frequency follows variation in resonance frequency caused by variation in the load of the vibrating-type compressor 1, and thus can drive the vibrating-type compressor 1 at maximum efficiency.
However, such a conventional drive apparatus may break, because it has mechanical contacts within the automatic changeover unit. Further, during AC input, conversion of ACxe2x86x92DCxe2x86x92DCxe2x86x92AC is performed, conversion efficiency is low, and therefore a relatively large amount of electrical power is consumed.
The inverter of the conventional drive apparatus outputs voltage which assumes a square waveform such that the signal assumes an on level potential during a first 180xc2x0-phase period within each period of the waveform and an off level potential during a second 180xc2x0-phase period within each period of the waveform. Since the voltage applied to the vibrating-type compressor 1 assumes not a sinusoidal waveform but a square waveform having a 180xc2x0 positive period and a 180xc2x0 negative period, the operation efficiency of the vibrating-type compressor 1 has been low.
When the above-described power supply apparatus is used, an AC voltage signal whose potential changes between positive and negative with respect to the zero potential is applied to the vibrating-type compressor. Therefore, one terminal of the vibrating-type compressor can be grounded, thereby enabling a cord to be connected to the casing itself of the vibrating-type compressor. However, a polarity inversion circuit as described has been required, in order to enable connection of a cord to the casing.
In view of the foregoing, an object of the present invention is to provide a drive apparatus for a vibrating-type compressor which solves the above-described problem.
Specifically, a first object of the present invention is to provide a drive apparatus for a vibrating-type compressor which employs a configuration which eliminates a mechanical changeover unit for effecting changeover between AC and DC and in which a commercial AC power source is connected to an inverter unit via an AC/DC converter and a diode OR circuit in order to eliminate mechanical contacts from the power system to thereby decrease failure rate; i.e., to improve reliability.
A second object of the present invention is to provide a drive apparatus for a vibrating-type compressor in which in place of conversion of ACxe2x86x92DCxe2x86x92DCxe2x86x92AC, conversion of ACxe2x86x92DCxe2x86x92AC is effected during AC input in order to improve conversion efficiency and reduce power consumption.
A third object of the present invention is to provide a drive apparatus for a vibrating-type compressor in which FETs disposed in the upper and lower arms of the inverter are turned on alternately such that each FET is in an on state over a 100xc2x0 to 140xc2x0 phase angle, whereby as compared with 180xc2x0 alternating supply of electricity, a wave closer to a sinusoidal wave is supplied to the vibrating-type compressor to thereby improve the operation efficiency of the vibrating-type compressor itself.
A fourth object of the present invention is to provide a drive apparatus for a vibrating-type compressor in which the on period over a 100xc2x0 to 140xc2x0 phase angle is slightly changed on the basis of the detected ambient temperature of the vibrating-type compressor, whereby the efficiency is improved further, and a valve-hitting phenomena peculiar to vibrating-type compressors is prevented.
A fifth object of the present invention is to provide a drive apparatus for a vibrating-type compressor which can cope with a wide range of input voltage, including AC 100 V, AC 200, DC 12 V, and DC 24 V, and which can detect a drop in the power source voltage regardless of the input voltage changing in the wide range.
A sixth object of the present invention is to provide a drive apparatus for a vibrating-type compressor which enables one end of the vibrating-type compressor to be grounded without provision of a polarity inversion circuit for generating a negative power source.
A seventh object of the present invention is to provide a drive apparatus for a vibrating-type compressor which enables reliable and accurate detection of the difference between a first-half peak and a second-half peak of a current waveform regardless of variation in frequency.
A drive apparatus for a vibrating-type compressor according to the present invention comprises a DC power source; an inverter 6 including switching elements and adapted to convert direct current to alternating current through alternating switching of the switching elements; and an inverter control unit 7 for controlling the alternating current supplied from the inverter 6 to the vibrating-type compressor 1. The DC power supply comprises a first power supply including a DC/DC converter 17 for converting direct current from a battery to direct current of a different voltage; a second power supply including an AC/DC converter 8 for converting alternating current from a commercial AC power source to direct current; a diode OR circuit (diodes 25 and 26) to which are connected the first and second power supplies; and a circuit which, upon detection of a voltage output from the second power supply, operates to stop the output of the first power supply via a DC/DC converter control unit 27 for controlling the DC/DC converter 17.
The drive apparatus for a vibrating-type compressor according to the present invention is preferably configured in such a manner that one of two output terminals of the first power supply and one of two output terminals of the second power supply are grounded; the grounded terminals are connected to one end of a first switching element (FET 5) of the inverter 6, while the other end of the first switching element (FET 5) is connected in series to one end of a second switching element (FET 4); and a voltage output from the OR circuit is supplied to the other end of the second switching element (FET 4). A current detection resistor (R) is connected in series to one of the two switching elements (FETs 4 and 5). The connection point between the two switching elements (FETs 4 and 5) is connected to one end of the vibrating-type compressor 1 via a capacitor 19, and the one end of the first switching element (FET 5) is connected to the other end of the vibrating-type compressor 1. Thus, the other end of the vibrating-type compressor 1 can be grounded.
The inverter control unit 7 of the drive apparatus for a vibrating-type compressor according to the present invention is preferably configured in such a manner that the inverter control unit 7 controls the on period within each switching cycle of the alternating switching operation of the switching elements within a phase angle range of 100xc2x0 to 140xc2x0 and that the on period of 100xc2x0 to 140xc2x0 is varied on the basis of a detected ambient temperature of the vibrating-type compressor.
The inverter control unit 7 of the drive apparatus for a vibrating-type compressor according to the present invention is preferably configured in order to detect and hold a first peak of current flowing during a half cycle of an AC output in which the first or second switching element enters a conductive state alternately, detect a timing at which the current reaches the held peak, and control the timing of the alternative switching of the two switching elements of the inverter to thereby variably control the output frequency of the inverter.
Further, the drive apparatus for a vibrating-type compressor according to the present invention preferably comprises a constant voltage generation circuit 28 for supplying a constant voltage for control use to the inverter control unit 7 and the DC/DC converter control unit 27; and an AC/DC changeover unit 9 which supplies direct current from the battery 2 to the constant voltage generation circuit 28 and supplies output from the AC/DC converter 8 to the constant voltage generation circuit 28 when the commercial AC power source is connected to the drive apparatus.